It has been observed by foresters that there is a tendency in any soil
which has long been occupied by a certain kind of timber, to produce,
after the trees have been felled, a very different kind, if it be left
to its spontaneous action. The laws affecting such rotations have been
very well ascertained, and a careful investigation of the subject would
undoubtedly reveal many curious facts not yet known. If the stumps of
the trees, consisting of oak, ash, maple, and some other deciduous
kinds, remain after the wood is felled, they will throw up suckers, and
the succeeding timber will be an inferior growth of the original wood.
But if the stumps and roots of the trees should be entirely removed, it
would be more difficult to determine what would be the character of the
next spontaneous growth. It would probably be planted by the kinds that
prevail in the neighboring forests, and it would depend on the character
of the soil whether the hard or soft-wood trees would finally
predominate.
There is an important chemical agency at work, that originally
determines the distribution of forests, and afterwards their rotation.
The hard-wood trees require more potash and a deeper soil than the
coniferous and soft-wood trees. Hence they are found chiefly on alluvial
plains and the lower slopes of mountains, where the soil is deep and
abounds in all valuable ingredients for the support of vegetation. Pines
and firs, on the contrary, though frequently discovered of an immense
size on alluvial soils, are generally crowded out of such grounds by the
superior vigor of the hard-wood trees; and they can only maintain their
supremacy on barren and sandy levels, and the thin soils of mountain
declivities, too meagre to support the growth of timber of superior
kinds. But a wood must stand a great many years, several centuries
perhaps, after its spontaneous restoration, before this order of nature
could be fully established. We must observe the spontaneous growth and
distribution of herbaceous plants in different soils to ascertain these
laws, which are the same in a field as in a forest.
When any growth of hard wood has been felled and the whole removed from
the ground, the soil, having been exhausted of its potash, cannot
support a new and vigorous growth of the same kind of timber. The
succession will consist of a meagre growth of the same species from
seeds already planted there; but the white birch and poplar, especially
the large American aspen, usually predominate in clearings in this part
of the country. When a pine wood is felled, it is succeeded by an
inferior growth of conifers, and a species of dwarf or scrub oak.
Seldom, indeed, after any kind of wood has been cut down and carried
away from the spot, can the exhausted soil support another that is not
inferior in quality or species. Though an oak wood may be succeeded by
pines, a pine wood will not be succeeded by oaks or any other hard
timber, unless the trees were burned and their ashes restored to the
soil. Hence we may account for the fact that poplars, white birches, and
wild-cherry-trees, occupy a larger proportion of the ground that is now
covered with wood than they did a century ago, in all parts of the
country.
I have already alluded to the well-known fact, that the generic
character of the timber, in the distribution of the primitive forest, in
any country, is determined in great measure by the geological character
of the soil. On sandy plains in the primitive forest, the white birch,
the poplar, the aspen, and the pitch pine were abundant, as they are now
on similar soils. The preference of the red maple for wet and miry soils
is well known; while hard maple, oak, beech, and hickory do not prosper
except in strong alluvial tracts. A heavy growth of hard timber
indicates a superior soil; pine indicates an inferior one, if it has
been left to the spontaneous action of nature. In the primitive forest
we were sure of finding such relations of soil and species. They are not
so invariable since the operations of agriculture have interrupted the
true method of nature.
When a wood has been burned, the process of renewal, when left to
nature, is much more tardy than if it had been felled, since it can now
be restored only by a regular series of vegetable species, which must
precede it, according to certain inevitable laws. The soil, however,
being improved and fertilized by the ashes of the burnt timber, is in a
chemical condition to support a luxuriant forest as soon as in the
course of nature it can be planted there. Trees will not immediately
come up from this burnt ground as in a clearing; and if they should
appear, they would mostly perish from the want of protection. In the
order of nature herbaceous plants are the first to occupy the soil, and
these are followed by a uniform succession of different species. There
is an epilobium, or willow herb, with elegant spikes of purple flowers,
conspicuous in our meadows in August, which is one of the earliest
occupants of burnt ground, hence called fireweed in Maine and Nova
Scotia. The downy appendage to its seeds causes it to be planted there
by the winds immediately after the burning. The trillium appears also in
great abundance upon the blackened surface of the ground in all wet
places. Plants like the ginseng, the erythronium, and the like, whose
bulbs or tubers lie buried deep in the mould, escape destruction, and
come up anew. These, along with several compound plants with downy
seeds, and a few ferns and equisetums, are the first occupants of burnt
lands.
But the plants mentioned above have no tendency to foster the growth of
young trees. They are, however, succeeded by the thistles and thorny
plants, which are nature’s preparation of any tract, once entirely
stripped of vegetation, as a nursery for the seedlings. All the
phenomena of nature’s rotation are but the necessary giving place of
rapid-growing and short-lived plants to others which are perennial and
more capable of maintaining their ground after being once planted.
Thorns and thistles soon appear on burnt lands, and protect the young
trees as they spring up, both from the winds and the browsing of
animals. Thus many an oak has been nursed in a cradle of thorns and
brambles, and many a lime-tree growing in a bower of eglantine has been
protected by its thorns from the browsing of the goat.
We very early discover a variety of those woody plants that bear an
edible fruit, which is eaten by birds and scattered by them over the
land, including many species of bramble. The fruit-bearing shrubs always
precede the fruit-bearing trees; but the burnt land is first occupied by
those kinds that bear a stone-fruit. Hence great numbers of cherry-trees
and wild-plum-trees are found there, as the natural successors of the
wild gooseberry and bramble-bushes. These are soon mixed with poplars,
limes, and other trees with volatile seeds. But oaks, hickories, and the
nut-bearing trees must wait to be planted by squirrels and field-mice
and some species of birds. The nut-bearers, therefore, will be the last
to appear in a burnt region, for the little quadrupeds that feed upon
their fruit will not frequent this spot until it is well covered with
shrubbery and other vegetation. If the soil be adapted to the growth of
heavy timber, the superior kinds, like the oak, the beech, and the hard
maple, will gradually starve out the inferior species, and in the course
of time predominate over the whole surface.
When I consider all these relations between plants and animals, I feel
assured, if the latter were destroyed that plant their seeds, many
species would perish and disappear from the face of the earth. Nature
has provided, in all cases, against the destruction of plants, by
endowing the animals that consume their fruits with certain habits that
tend to perpetuate and preserve them. In this way they make amends for
the vast quantities they consume. After the squirrels and jays have
hoarded nuts for future use, they do not find all their stores; and they
sow by these accidents more seeds than could have been planted by other
accidental means, if no living creature fed upon them. Animals are not
more dependent on the fruit of these trees for their subsistence, than
the trees are upon them for the continuance of their species. And it is
pleasant to note that, while plants depend on insects for the
fertilization of their flowers, they are equally indebted to a higher
order of animals for planting their seeds. The wasteful habits of
animals are an important means for promoting this end. The fruit of the
oak, the hickory, and the chestnut will soon decay if it lies on the
surface of the ground, exposed to alternate dryness and moisture, and
lose its power of germination. Only those nuts which are buried under
the surface are in a condition to germinate. Many a hickory has grown
from a nut deposited in the burrow of a squirrel; and it is not an
extravagant supposition that whole forests of oaks and hickories may
have been planted in this manner.
These facts are too much neglected in our studies of nature. A knowledge
of them, and a consideration of their bearings in the economy of nature,
might have saved many a once fertile country from being converted into a
barren waste, and may serve yet to restore such regions to their former
happy condition. But these little facts are not of sufficient magnitude
to excite our admiration, and they involve a certain process of
reasoning that is not agreeable to common minds, or even to the more
cultivated, which have been confined chiefly to technology. The few
facts to which I have alluded in this essay are such as lie at the
vestibule of a vast temple that has not yet been entered. I am not ready
to say that no single species of the animal creation may not be
destroyed without derangement of the method of nature; for thousands
have, in the course of time, become extinct by the spontaneous action of
natural agents. But there is reason to believe that, if any species
should be destroyed by artificial means, certain evils of grievous
magnitude might follow their destruction.
The frugivorous birds are the victims of constant persecution from the
proprietors of fruit gardens. Their persecutors do not consider that
their feeding habits have preserved the trees and shrubs that bear fruit
from utter annihilation. They are the agents of nature for distributing
vegetables of all kinds that bear a pulpy fruit in places entirely
inaccessible to their seeds by any other means. Notwithstanding the
strong digestive organs of birds, which are capable of dissolving some
of the hardest substances, the stony seeds of almost all kinds of pulpy
fruit pass through them undigested. By this providence of nature the
whole earth is planted with fruit-bearing trees, shrubs, and herbaceous
plants, while without it these would ultimately become extinct. This may
seem an unwarrantable assertion. It is admitted that birds alone could
distribute the seeds of this kind of plants upon the tops of mountains
and certain inaccessible declivities, which, without their agency, must
be entirely destitute of this description of vegetation. But these
inaccessible places are no more dependent on the birds than the plains
and the valleys. The difference in the two cases is simply that the one
is apparent, like a simple proposition in geometry, and the other
requires a course of philosophical reasoning to be perfectly understood.